Power circuit

Abstract

To provide a power supply circuit capable of preventing a failure at the detachment of a load.SOLUTION: A power supply circuit 1 comprises: a microcomputer 5 that controls FETs 7a and 7b; and a resistance 10 that detects a current flowing in an LED 20. The microcomputer 5 determines whether or not the LED 20 has been detached on the basis of a current value detected by the resistance 10. When it is determined that the LED 20 has been detached, the FET 7b is set to ON for a predetermined time to discharge a capacitor 9.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a power supply circuit that supplies power to, for example, LED lighting. [Background technology]

[0002] In recent years, light-emitting diodes (LEDs), which consume low power, have been used as light sources in lighting fixtures. LEDs are generally driven by a constant current, so the power supply circuit that supplies power to the LED is a constant current power supply.

[0003] In this type of power supply circuit, Patent Document 1 describes that if the LED of the light source fails open, the light source current becomes zero and the smoothing capacitor only charges but does not discharge, causing the voltage across the LED to increase, and this state is judged to be an abnormality. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-165546 Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 describes that when the abnormality determination unit determines that an abnormality has occurred, the signal generation unit does not generate a control signal, causing the drive circuit to not generate a drive signal, and the two switching elements continue to remain in the off state, thereby stopping the generation of AC voltage, and therefore the lighting device safely shuts down.

[0006] However, even if the two switching elements continue to be in the off state and the generation of AC voltage is stopped, charge accumulates in the smoothing capacitor at the output terminal, and the smoothing capacitor will discharge due to natural discharge. Also, not limited to the circuit configuration of Patent Document 1, in switching power supply circuits, it is common for a capacitor to be provided at the output terminal of the circuit for smoothing, and the natural discharge described above may take several seconds to about 10 seconds, depending on the capacity of the smoothing capacitor.

[0007] Furthermore, for example, in the case of fluorescent-type LEDs, replacement may be performed without turning off the power and while the LED is still on. During this replacement, if contact such as chattering occurs between the LED terminals and the socket while the smoothing capacitor is naturally discharging, a large current may flow through the LED due to the charge accumulated in the smoothing capacitor, potentially causing a malfunction of the LED load. Patent Document 1 does not take into consideration the possibility of malfunctions caused by such events.

[0008] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a power supply device that can prevent failure when a load is removed. [Means for solving the problem]

[0009] The invention described in claim 1, which was made to solve the above problem, is a power supply circuit comprising a switching element for supplying a constant current to a load and a control unit for controlling the switching element, wherein a capacitor is provided at the output end of the switching element, and wherein, when the control unit determines that the load has been removed, the control unit controls the switching element to be turned on for a predetermined time to discharge the capacitor. [Effects of the Invention]

[0010] According to the present invention, when the control unit determines that the load has been removed, it controls the switching element to be ON for a predetermined time to discharge the capacitor, thereby forcibly discharging the charge in the capacitor and preventing malfunction when the load is removed. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a circuit diagram of a power supply circuit according to an embodiment of the present invention; [Figure 2] 10A and 10B are waveform diagrams showing changes in output voltage and output current in a conventional power supply circuit. [Figure 3] 2 is a waveform diagram showing changes in output voltage and output current in the power supply circuit shown in FIG. 1. [Figure 4] FIG. 4 is a detailed waveform diagram of FIG. 3. [Figure 5] 2 is a flowchart of the operation of the microcomputer shown in FIG. 1. DETAILED DESCRIPTION OF THE INVENTION

[0012] An embodiment of the present invention will now be described with reference to Figures 1 to 5. Figure 1 is a circuit diagram of a power supply circuit according to an embodiment of the present invention.

[0013] The power supply circuit 1 includes a diode bridge 2, a PFC 3, a microcomputer power supply 4, a microcomputer 5, a FET driver 6, FETs 7a and 7b, an inductor 8, a capacitor 9, a resistor 10, and an operational amplifier 11. An LED 20 is connected to the power supply circuit 1 as a load.

[0014] The diode bridge 2 is a bridge circuit made up of well-known diodes for rectifying AC power, which is input from a commercial power source or the like, into DC power and outputting the DC power.

[0015] The PFC 3 is a general boost type power factor correction circuit that improves the power factor of the DC power rectified by the diode bridge 2 and supplies it to a subsequent stage.

[0016] The microcomputer power supply 4 is a power conversion circuit that converts the power supply voltage to be supplied to the microcomputer 5. The microcomputer 5 is a microcomputer equipped with a CPU (Central Processing Unit), ROM (Read Only Memory), RAM (Random Access Memory), etc., and operates according to a control program stored in the ROM. The microcomputer 5 is responsible for overall control of the power supply circuit 1. The microcomputer 5 also controls the switching of the FETs 7a and 7b based on the current value input from the operational amplifier 11. In other words, the microcomputer 5 functions as a control unit that controls the switching elements.

[0017] The FET driver 6 performs ON / OFF switching control of the FETs 7 a and 7 b under the control of the microcomputer 5 .

[0018] The FETs 7a and 7b are field-effect transistors. The FET 7a has a gate connected to the FET driver 6, a drain connected to the output of the PFC 3, and a source connected to the drain of the FET 7b and one end of the inductor 8.

[0019] The gate of FET 7b is connected to the FET driver 6, the drain is connected to the source of FET 7a and one end of inductor 8, and the source is connected to the output of PFC3, the other end of capacitor 9, one end of resistor 10, and the negative terminal of operational amplifier 11. The source of FET 7b is grounded. In other words, the lower output of PFC3 in FIG. 1 is grounded, and FET 7b serves as the low side (FET 7a serves as the high side).

[0020] The FETs 7a and 7b perform switching operations when their gates are controlled by the FET driver 6. That is, the FETs 7a and 7b function as switching elements. As is clear from the above description, the FETs 7a and 7b are connected in series.

[0021] One end of inductor 8 is connected to the source of FET 7a and the drain of FET 7b, and the other end is connected to one end of capacitor 9 and one end of LED 20. One end of capacitor 9 is connected to the other end of inductor 8 and one end of LED 20, and the other end is connected to the output of PFC3, the source of FET 7b, one end of resistor 10, and the negative terminal of operational amplifier 11. The other end of capacitor 9 is grounded, as is the source of FET 7b. Inductor 8 and capacitor 9 smooth the output voltage generated by FETs 7a and 7b. Capacitor 9 is also provided at the output end of FET 7b, which is a switching element as is clear from FIG. 1.

[0022] One end of resistor 10 is connected to the output of PFC3, the source of FET 7b, the other end of capacitor 9, and the negative terminal of operational amplifier 11, and the other end is connected to the other end of LED 20 and the positive terminal of operational amplifier 11. Operational amplifier 11 detects the current flowing across resistor 10. That is, resistor 10 and operational amplifier 11 function as a detector that detects the current flowing to the load.

[0023] The LED 20 is connected as a load to the power supply circuit 1. The LED 20 is configured as, for example, a fluorescent lamp (straight tube type) LED lighting. Therefore, the LED 20 and the power supply circuit 1 are detachable via a socket or the like.

[0024] Next, an example of the operation of the above-mentioned power supply circuit will be described. First, a conventional power supply circuit will be described with reference to Fig. 2. Fig. 2 shows waveforms showing changes in output voltage and output current in a conventional power supply circuit.

[0025] First, when the LED 20 is connected to the power supply circuit, both the output voltage V and the output current I remain constant. Next, when the LED 20 is removed from the power supply circuit (time t1), the output current I becomes "0" and the output voltage V rises. Because the LED 20 is normally driven with a constant current, when the LED 20 is removed, the power supply circuit tries to maintain a constant current by increasing the voltage. However, once the output voltage V rises to a certain voltage, a protection circuit or the like stops the switching operation. Then, the output voltage V gradually decreases due to natural discharge of the smoothing capacitor provided at the output end of the power supply circuit.

[0026] Here, if the LED 20 comes into contact with an output terminal of the power supply circuit or the like (at time t2) before the output voltage V falls below the forward voltage VF of the LED 20, a large current will flow through the LED 20. If such a large current flows, the LED 20 may break down.

[0027] Next, FIG. 3 shows waveforms showing changes in the output voltage and output current in the power supply circuit 1 according to this embodiment.

[0028] In the case of Figure 3, the situation is the same as in Figure 2 up to time t1. Then, when the output voltage V rises to a certain voltage and the switching operation of FETs 7a and 7b stops, the low-side FET 7b turns ON for a predetermined time (time t1-1). This forces the capacitor 9 to discharge, causing the output voltage V to drop sharply.

[0029] Therefore, even if the LED 20 comes into contact with the output terminal of the power supply circuit (at time t2), the output voltage V is equal to or lower than the forward voltage VF, so no large current flows through the LED 20. Therefore, it is possible to prevent the LED 20 from breaking down due to a large current.

[0030] Figure 4 shows a detailed waveform diagram of Figure 3. Figure 4 adds the waveforms of the low-side FET 7b and high-side FET 7a to Figure 3. The low-side FET 7b and high-side FET 7a alternate between ON and OFF at a predetermined cycle T to keep the output current I constant. In other words, they are switched. Here, the cycle of the low-side FET 7b and the cycle of the high-side FET 7a are out of phase with each other by half a cycle. In other words, when the low-side FET 7b is ON, the high-side FET 7a is OFF, and when the low-side FET 7b is OFF, the high-side FET 7a is ON.

[0031] 4, times t11, t13, and t14 are times when the microcomputer 5 samples the current value detected by the operational amplifier 11. At time t11, it is assumed that a value is obtained for the output current I to drive the LED 20. Then, at time t12, which is between times t11 and t13, it is determined that the LED 20 has been removed (load release), and the output current I becomes "0" at times t13 and t14, and the output voltage V begins to rise.

[0032] In this embodiment, the load disconnection detection condition is a change in the output current I. Specifically, (1) a negative slope of the output current is detected, and (2) a current value equal to or less than a predetermined value is detected twice in succession. An example of condition (1) is when the previously sampled output current value I(t-1) and the currently sampled output current value I (t) An example of condition (2) is when the sampled output current I is detected to be 50 mA or less twice in a row.

[0033] When the above conditions (1) and (2) are met, the microcomputer 5 determines that the load is open, and turns on the low-side FET 7b for a predetermined time (e.g., several tens of microseconds) while keeping the high-side FET 7a in the OFF state. After the predetermined time has elapsed, the low-side FET 7b is turned off. This forces the capacitor 9 to discharge, causing the output voltage V to drop sharply.

[0034] Therefore, even if the LED 20 comes into contact with an output terminal of the power supply circuit at time t15, the output voltage V is equal to or less than the forward voltage VF, so no large current flows through the LED 20. In this embodiment, it still takes time for the output voltage V to fall below the forward voltage VF, but this can be made very short by forced discharge (approximately several tens of microseconds in the example of FIG. 4), significantly reducing the possibility of a load breakdown due to a large current.

[0035] Figure 5 shows a flowchart of the operation of the microcomputer 5. First, it is determined whether removal of the load (LED 20) has been detected (step S1). The determination method is whether the above-mentioned conditions (1) and (2) are satisfied. That is, the microcomputer 5 (controller) determines whether the LED 20 (load) has been removed based on the current value detected by the resistor 10 and the operational amplifier 11 (detector).

[0036] If the removal of the load is detected (step S1; YES), the high-side FET 7a is kept OFF and the low-side FET 7b is turned ON for a predetermined time (step S2). That is, if the microcomputer 5 (controller) determines that the LED 20 (load) has been removed, it controls the FET 7b (switching element) to be ON for a predetermined time to discharge the capacitor 9.

[0037] According to this embodiment, the power supply circuit 1 includes a microcomputer 5 that controls the FETs 7a and 7b, and a resistor 10 that detects the current flowing through the LED 20. The microcomputer 5 determines whether the LED 20 has been removed based on the current value detected by the resistor 10 and the operational amplifier 11, and if it determines that the LED 20 has been removed, it controls the FET 7b to be ON for a predetermined time to discharge the capacitor 9. This allows the charge in the capacitor 9 to be forcibly discharged, preventing damage to the LED 20 due to chattering or the like when the LED 20 is removed.

[0038] Furthermore, since whether the LED 20 has been removed is determined based on the current value detected by the resistor 10 and the operational amplifier 11, it is possible to determine whether the LED 20 has been removed by using, for example, a resistor provided for constant current control. Therefore, it is possible to determine whether the LED 20 has been removed without the need for an additional circuit.

[0039] Furthermore, FETs 7a and 7b are connected in series, and the microcomputer 5 controls the low-side FET 7b to be turned on for a predetermined time to discharge the capacitor 9. In this way, by controlling the grounded FET 7b to be turned on, a closed circuit for discharging the capacitor 9 is formed, enabling forced discharge.

[0040] Furthermore, since the load is the LED 20, it is possible to reduce failures due to chattering that may occur when replacing a straight tube LED, for example.

[0041] In the above-described embodiment, the switching element is described as an FET, but this is not limiting. For example, other switching elements such as transistors may be used. Furthermore, the load is not limited to an LED light, and may be any device that is driven by a constant current, such as an organic EL (electroluminescence) light.

[0042] Furthermore, the present invention is not limited to the above-described embodiments. That is, those skilled in the art can implement various modifications in accordance with conventionally known knowledge without departing from the gist of the present invention. As long as such modifications still include the configuration of the power supply circuit of the present invention, they are of course included in the scope of the present invention. [Explanation of symbols]

[0043] 1 Power circuit 5 Microcomputer (control unit) 7a, 7b FET (switching element) 9. Capacitors 10 Resistor (detection part) 11 Operational amplifier (detection section) 20 LEDs (load)

Claims

1. A power supply circuit comprising two switching elements connected in series to supply a constant current to a load, and a control unit that controls the two switching elements, and further comprising a capacitor and an inductor connected in series, the capacitor is connected in parallel with the load between the inductor and ground; the series-connected capacitor and inductor are connected in parallel with a low-side switching element of the two switching elements between a high-side switching element of the two switching elements and ground; When the control unit determines that the load has been removed, the control unit controls the low-side switching element to be ON for a predetermined time to discharge the capacitor.

2. a detection unit that detects a current flowing through the load, 2. The power supply circuit according to claim 1, wherein the control unit determines whether the load has been removed based on the current value detected by the detection unit.

3. 3. The power supply circuit according to claim 1, wherein the load is an LED light.

Images